The present invention relates to a novel and cost effective process for converting sour natural gas which may also be static and poor, i.e. natural gas which containing hydrogen sulfide and which may also be remotely located and contain substantial amounts of nitrogen and/or carbon dioxide, to useable fuels and chemicals, such as hydrogen, methanol and high octane diesel fuel. More particularly, the present invention relates to a method and apparatus for treating xe2x80x9csour natural gas, i.e., gas having a ratio of H2S to CH4 of at least 0.1 moles H2S per mole CH4 and preferably of at least 0.5 moles/mole, using a reforming catalyst and a sulfur capture agent. Preferably, the process according to the invention can be carried out using two reactors that repeatedly cycle reactants between three basic process stepsxe2x80x94reforming, air regeneration and fuel reduction.
A large fraction of the world""s total natural gas reserves has the problem of being xe2x80x9csourxe2x80x9d in that they contain substantial amounts of H2S, which is both highly toxic and tends to embrittle steel pipelines, making the transport of gases by pipeline highly dangerous and unreliable due to the possibility of leakage in the gas lines and transport equipment. Much of world""s total natural gas reserves also has the problem of being xe2x80x9cstatic,xe2x80x9d i.e., the gas is located in remote geographic regions that make it uneconomical to transport the gas via pipeline or to refine and/or condense the gas on site and ship it to market in liquid form. The world""s total natural gas reserves also include much that is poor in quality because the methane and other combustible gas components are diluted with non-combustible CO2 and N2, making the unrefined gas a relatively low Btu fuel source.
Thus, for many years, the need has existed to convert sour natural gas which may also be static and/or poor into a more valuable commercial product which could then be transported in large quantities by inexpensive means (preferably by ship). The current state of industrial practice with sour natural gas that is also static and poor is illustrated by Exxon""s development of the Natuna gas fields located in the middle of the South China Sea. Because the natural gas deposits contain high percentages of CO2 and H2S, the gas is considered both poor and sour. In this project the CO2 and H2S are removed by liquefying and fractionally distilling the gas. This approach, while technically feasible, is very expensive. The static gas problem was resolved by developing a local use for the gas on site, namely as a fuel for use in producing steam for secondary oil recovery in the same remote geographic location. The Exxon approach made good economic sense because it began with two low value natural resources (a static, poor quality sour gas field and a depleted oil field) and finished with a relatively high quality crude oil end product using secondary oil recovery techniques.
In principle, it is possible to use fractional distillation processes to purify poor sour gas, followed by a conventional steam reforming process to convert the purified gas into a mixture of CO and hydrogen (commonly known as xe2x80x9csyngasxe2x80x9d). The syngas can then be converted into liquid hydrocarbons via the well-known Fischer-Tropsch process or to other commercially useful products such as methanol. This conventional gas reforming approach, however, has a number of significant disadvantages.
For example, liquefaction and fractional distillation of natural gas consumes a great deal of energy as part of the process, and thus requires a high capital expense for the equipment necessary to carry out such techniques. In addition, since steam reforming is economical only in large scale applications, a given gas resource may not be large enough to sustain the cost of a steam reformer. Furthermore, steam reforming is an endothermic reaction and in the conventional steam reforming process the heat consumed by the reaction is supplied by heating the outside of the reactor. This requires that the walls of the reactor be at a temperature equal to or greater than the temperature at which the steam reforming reaction occurs. This temperature is substantially above the temperature at which conventional steels begin losing their mechanical strength and resistance to corrosion.
As a result, conventional steam reforming normally requires processing equipment formed from expensive superalloy metals. The operating pressure at which the steam reforming process takes place must also be reduced. That is, even though the natural gas issuing from a well head may be at a pressure high enough for use in the Fischer-Tropsch process, the chemical composition of such gases does not allow for their direct use with Fischer-Tropsch. Thus, although steam reforming can produce gas of the appropriate composition, the reforming process requires first depressurizing the gas. Compressing the syngas back up to the pressure needed for the Fischer-Tropsch process can be prohibitively expensive.
In addition to these specific disadvantages, a general problem exists with conventional steam reforming processes in that many sour gas resources are found in regions of the world which lack the infrastructure necessary to support complex industrial processes. By necessity, the only practical industrial operations in such regions are those which are relatively simple to install, operate and maintain. Further, if a natural gas resource is poor in quality because it contains substantial amounts of nitrogen but not a significant amount of CO2, no economically viable process exists within the present state-of-the-art to easily purify the gas on site. The only known approach is to treat the entire gas stream and remove the nitrogen using conventional (but very expensive) processing means.
On the other hand, if the natural gas is poor in quality because of a high rather than low CO2 content, then in some situations the CO2 can be put to advantageous use. It is known that nickel-based catalysts used in steam reforming can also be used with CO2 in the reforming of methane. An article by Tomishge et al, Catalysis Today 45, 35-39, (1988) discusses the CO2 reforming of CH4 and notes the advantage of utilizing both the CO2 and CH4 components of the natural gas. While nickel-based catalysts are the most widely used reforming catalysts, the literature reports a number of other noble metal catalysts that can serve as active reforming catalysts (see, for example, the article by Craciun et al, Catalysis Letters 51, 149-151, 1998).
Prior art U.S. Pat. No. 5,827,496 teaches a method of supplying heat to packed bed reactors which are used to carry out endothermic reactions such as steam reforming. In this method, heat is generated inside the reactor by alternately reducing and oxidizing a material which in the reduced state is readily oxidized and in the oxidized state is readily reduced. This method is called xe2x80x9cAutothermal Cyclic Reforming (ACR) or xe2x80x9cUnmixed Reformingxe2x80x9d (UMR).
The examples in the ""496 patent show the production of hydrogen by steam reforming with a nickel catalyst in the presence of CaO. The CaO captures CO2 by forming CaCO3 in an exothermic process. While the CaCO3 formation supplies the heat consumed by the steam reforming process, CaCO3 must be regenerated in an endothermic process. In order to supply the heat necessary to regenerate the CaCO3, air is passed through the bed, oxidizing the nickel catalyst to NiO in a strongly exothermic process. The NiO is then reduced back to nickel and the production of hydrogen is resumed. Example 8 of the ""496 patent describes a process for steam reforming diesel fuel to which thiophene has been added to a level of 2000 ppm sulfur by weight, producing an output hydrogen gas containing only 5 ppm H2S. Thus, the ""496 patent teaches that the process could produce hydrogen that is significantly lower in sulfur content than the fuel being reformed.
The process described in the ""496 patent has two significant limitations. First, the patent is concerned solely with the reforming of materials in which the sulfur content is very low, i.e., an impurity and/or minor component. In sour gas, however, sulfur in the form of H2S is often a major constituent. Second, the ""496 reference is limited to methods of generating heat within a reactor in which by the sequential oxidation and reduction of a material which in the reduced state is readily oxidized and which in the oxidized state is readily reduced. In no wise does ""496 teach show or suggest the generation of heat by selectively oxidizing one low value component (i.e. the H2S) of the natural gas while retaining the CH4 and other high value components.
The steam reforming of a fuel is an endothermic reaction. Supplying the heat which is consumed by this reaction requires either burning some other fuel or burning part of the fuel being processed. In addition to the heat consumed directly by the steam reforming reaction doing steam reforming requires raising steam. This steam raising also requires either burning some other fuel or burning part of the fuel being processed. A sour natural gas is a mixture of fuels, the methane and other hydrocarbons it contains being valuable components and the H2S being of little or negative worth. Oxidation of the H2S will, however, liberate substantial amounts of heat.
It is a goal of the present invention to provide a means whereby the heat of oxidation of H2S is used to supply part or all of the heat consumed by the steam reforming of the natural gas. It is a further goal of the present invention to do said supplying of heat in a manner that allows the steam reforming reaction to be done at high pressure. Said high pressure operation reduce or eliminates the need to compress the gases produced by steam reforming before putting them to some use, e.g. producing hydrocarbons via the Fischer-Tropsch process. It is a still further goal of the present invention to minimize or completely avoid air polluting emissions of sulfur dioxide, to provide some or all of the heat needed to raise steam without burning methane or other high value components of the natural gas, to provide some or all of the electrical energy needed without burning methane or other high value components of the natural gas.
Unlike the above prior art, the present invention provides a novel and cost-effective means whereby sour gas (which may also be poor and static) is converted to liquid hydrocarbons, methanol, and/or other valuable fuel materials that can readily be treated on site and then transported to market in a safe and reliable manner using conventional steel pipelines. The preferred method according to the invention has been found to be most effective for treating sour gas having a ratio of H2S to CH4 of at least 0.1 moles H2S per mole CH4 and preferably at least 0.33 moles/mole.